Photo-crosslinkable polyolefin compositions

ABSTRACT

A photo-crosslinkable polyolefin composition comprises a polyolefin, a source of functionality receptive to crosslinking by UV radiation, a cationic photoinitiator and optionally includes a free-radical photoinitiator, a crosslinking accelerator or sensitizer, and other additives such as compatibilizers, inorganic fillers, nanofillers, glass, polymeric and ceramic microspheres, glass fibers, flame retardants, antioxidants, stabilizers, processing aids, foaming agents and pigments. A method for manufacturing a UV-crosslinked polyolefin article comprises forming an article by extruding, molding or otherwise forming the UV-crosslinkable polyolefin composition and subjecting the article to UV radiation on-line with the extrusion, molding or other forming operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 11/680,068,filed Feb. 28, 2007, now U.S. Pat No. 7,744,803.

FIELD OF THE INVENTION

The present invention relates to polymer compositions, articles madetherefrom, and methods for the production and processing of thesecompositions and articles. More particularly, the compositions accordingto the invention are polyolefin-based compositions and are crosslinkableby exposure to ultraviolet radiation. The articles according to theinvention are coatings and insulating materials. The invention allowson-line crosslinking of the polymer during production of the articles.

BACKGROUND OF THE INVENTION

Due to an attractive balance of performance and cost, polyolefin resins,such as polyethylenes, polypropylenes, copolymers of ethylene andpropylene, and compositions based thereon, are widely used in coatingand insulation applications. These applications include: heat-shrinkablecorrosion-protection sleeves for oil and gas pipeline joints; solid andfoamed coatings for the corrosion, mechanical and thermal protection ofpipelines and pipeline structures; wire and cable insulations andjacketing; and heat-shrinkable extruded tubing or molded shapes for theelectrical insulation and mechanical protection of wires, cables,connectors, splices and terminations.

Many of these applications require that the coating or insulatingmaterial provide acceptable thermal and mechanical performance attemperatures close to or above the softening or melting point of thethermoplastic polyolefin resin(s) from which it is made. Suchperformance requirements include, but are not limited to, long-termcontinuous operating temperature, hot deformation resistance, hot settemperature, chemical resistance, tensile strength and impactresistance. To achieve these requirements it is necessary to impart somethermoset characteristic to the resin or polymer. This is accomplishedby crosslinking the molecular structure of the polymer to some requireddegree. Crosslinking renders the material resistant to melting andflowing when it is heated to a temperature close to or above thecrystalline melting point of the highest melting point polymericcomponent of the composition. This property is also necessary for theproduction of heat-shrinkable articles, such as pipe joint protectionsleeves, where crosslinking imparts controlled shrinkage, or heatrecovery, characteristics, and prevents the material from melting whenit is heated to the temperature necessary to effect heat recovery.

Crosslinking, or curing, of polyolefin-based coatings or insulatingmaterials is typically accomplished through one of two basic methods: byirradiation, such as exposure to electron beam radiation; or bythermo-chemical reaction, such as that induced by peroxide decompositionor silane condensation. The advantages and disadvantages of thesemethods are noted below.

Irradiation of the polymer by electron beam generates free-radicals onthe polymer chains which then covalently combine to effect crosslinkingof the polymer. It is an instantaneous and clean method, but requiresexpensive, and potentially dangerous, high voltage “electron-beam”equipment. It also has limitations in terms of the product thickness andconfiguration that can be crosslinked uniformly.

Peroxide crosslinking is also a free-radical process but here thefree-radicals are chemically generated through decomposition of theperoxide by heat. The process is thickness independent but needssubstantial amounts of heat to effect crosslinking, is performed atrelatively low processing speeds, and is frequently coupled withcumbersome and expensive processing equipment, such as pressurized steamor hot-gas caternary lines. A major disadvantage of using the hightemperatures required to induce peroxide crosslinking (typically 200 to350° C.) is potential softening, damage, and oxidative degradation ofthe polymer.

Silane crosslinking, also known as moisture crosslinking, occurs viahydrolysis and condensation of silane functionality attached to thepolymer to be crosslinked. It is a relatively inexpensive process butrequires a preliminary silane grafting or copolymerization operation,has restrictions in terms of polymer formulation flexibility, and isvery time dependent, requiring many hours or days in a hot, moistenvironment to achieve full crosslinking of the polymer.

Typically, the crosslinking operations described above are performed asseparate and discrete processes subsequent to melt processing, orforming, of the polymer article. It is, however, advantageous in termsof production efficiency, product throughput, and operating cost toperform the crosslinking operation at the same time as, and on-linewith, the polymer processing, or forming, operation, and immediatelyfollowing solidification of the formed article.

Of the methods described above, only the peroxide method realisticallyprovides the opportunity of crosslinking in situ or “on-line” with thepolymer processing or forming operation. The size, complexity, andsafety risks of an electron beam typically preclude its use as anon-line crosslinking device. In the case of silane crosslinking, thecrosslinking reaction can only be accomplished off-line since it is ahighly time-dependant reaction, influenced by the diffusion of moistureinto the polymer.

Crosslinking using ultra-violet (UV) radiation, namely radiation in therange of 200 to 500 nanometers wavelength, and also known asphoto-crosslinking, provides a potential solution to the problemsdescribed. Compared with electron beam irradiation, the UV sourcerequired to effect crosslinking is relatively small, more easilyconfigurable, less expensive and safer to use. It offers the potentialof a portable crosslinking device which can be moved into positiondownstream of the polymer melt processing, or forming, operation. Forexample, the device may be positioned between an extruder and a producthandling, or wind-up, station of a continuous polymer extrusion process,to allow on-line crosslinking of an extruded article, such as sheet,tubing, or wire insulation.

There are two primary methods of crosslinking or polymerization using UVradiation: free-radical and ionic.

UV free-radical crosslinking results from a reaction involving aphotoinitiator, such as benzophenone, benzyldimethylketal andacylphosphine oxides, which absorbs UV light to dissociate into freeradicals which can then initiate the crosslinking or polymerizationreaction. A multifunctional crosslinking agent, such as triallylcyanurate or trimethylolpropane triacrylate, may be additionallyincorporated to achieve higher levels of crosslinking.

Unfortunately, a major disadvantage of UV free-radical crosslinking hasbeen that it cannot readily be used for crosslinking thick or solidpolymer sections, such as the functional thicknesses required for thepipe coatings, heat-shrinkable coverings, and wire and cable insulationsdescribed above. This is because of the relatively weak intensity of UVlight which results in poor penetration of the radiation through thesolid material, compared with electron beam radiation, for example. Thisis particularly the case with semi-crystalline polymeric materials, suchas polyolefins, where the dense crystalline regions are relativelyimpenetrable to UV radiation. The effectiveness of UV free-radicalcrosslinking is also compromised if the resin to be crosslinkedcomprises additional materials such as filler and stabilizer additives,since these can provide further barriers to penetration by the UV lightas well as interfering with the crosslinking reaction by neutralizingthe free-radicals required for crosslinking. In addition, UVfree-radical crosslinking is severely inhibited by the presence ofoxygen, and for this reason is ideally performed in an inert atmosphere,such as nitrogen.

Traditionally, therefore, the use of UV free-radical crosslinking hasbeen restricted to the curing or polymerization of liquid or lowviscosity functional monomers or oligomers, such as acrylates,methacrylates and unsaturated polyesters, in thin (typically less than0.250 mm., more typically less than 0.100 mm.) coating applications,such as film coatings, paints, inks, and varnishes, or for sealants andpressure sensitive adhesives, whereby the liquid or low viscositymonomers or oligomers are converted to a solid or gel-like material.

UV crosslinking by ionic reaction, that is anionic or cationicpolymerization, and more particularly cationic polymerization, hashistorically found limited use compared with the UV free-radical processdue to the unavailability of effective cationic photoinitiators.However, recent technical advances in cationic photochemistry are nowmaking this technique more attractive for commercial applications. Theprocess relies on the cationic polymerization of epoxy, oxetane, vinylether and similar functionalities by strong protonic acids created bythe UV irradiation of onium salts, such as aryldiazonium salts,triarylsulphonium and diaryliodonium salts, for example. The first typegenerates Lewis acids whilst the last two types produce Bronsted acids,these being preferable as initiating entities for cationicpolymerization.

A very useful feature of cationic polymerization is that the reaction ismostly thickness independent and will continue to proceed to completion“in the dark” after the UV source has been removed. In addition, thecationic photoinitiation reaction is not inhibited by oxygen as isfree-radical photoinitiation.

An example of a typical cationic reaction mechanism is shown below inrelation to the polymerization of a cycloaliphatic epoxide.

Reaction Step 1: On UV irradiation, the cationic photoinitiatorinteracts with active hydrogen naturally present to produce a strongprotonic, or Bronsted, acid, and various aryl sulphur compounds:

Reaction Step 2: The acid will protonate epoxy, or oxirane, groups, andpolymerization then proceeds by ring-opening reaction:

PRIOR ART

European Patent 0490854A2 describes one attempt to address the problemof crosslinking relatively thick extruded polyethylene materials by UVradiation (in this case an extruded strip of thickness 0.5 mm.). Aproprietary benzophenone free-radical photoinitiator having low vapourpressure and high polymer solubility is used in combination with acrosslinking promoter to effect rapid crosslinking of extrudedpolyethylene. However, due to the problems associated with UVpenetration described earlier, the crosslinking operation needs to becarried out in the melt, in other words before the polymer hassolidified or crystallized. This severely restricts the use of thismethod in most extrusion operations, where it is necessary to shape thepolymer and cool the material below its melting point immediately afterexiting the extruder die. Crosslinking of the polymer in the melt statenecessarily fixes the shape of the extrudate or dramatically increasesthe material viscosity, thereby limiting any downstream sizing orshaping operations that may need to be performed. It is also practicallyvery difficult to insert a UV radiation device between the extruder dieand adjacent cooling equipment, such as a water trough or casting stack,without severely impeding the overall extrusion operation.

Japanese Patent Application 05024109A2 uses a similar free-radicaltechnique to crosslink an extruded polyolefin tube which is thenexpanded to create a heat-shrinkable tubular product. Again this processis performed in the melt state, so the limitations described aboveremain unaddressed.

SUMMARY OF THE INVENTION

The present invention overcomes the above-mentioned deficiencies of UVcrosslinking and the above-mentioned prior art by providing a meanswhereby extruded, moulded or formed polyolefin and polyolefin-basedmaterials, of the functional thicknesses required for applications suchas pipe coatings, heat-shrinkable coverings and wire and cableinsulations, can be crosslinked in the solid state. In addition,crosslinking is not restricted to being performed as a separateoperation subsequent to, the extrusion, moulding or forming operation.

In one aspect, the present invention provides UV-crosslinkablepolyolefin compositions comprising a polyolefin; a source offunctionality receptive to crosslinking by UV radiation, preferably apolymer, and more preferably a polyolefin, copolymerized or grafted withsaid functionality, where said functionality is cationicallypolymerizable, or a combination of cationically and free-radicallypolymerizable functionalities; a cationic photoinitiator; an optionalfree-radical photoinitiator; an optional crosslinking accelerator orsensitizer; and optional additives such as compatibilisers, inorganicfillers, nanofillers, glass and ceramic microspheres, glass fibres,flame retardants, antioxidants, stabilizers, processing aids, foamingagents, peroxides, and pigments.

In another aspect, the present invention provides a method formanufacturing a UV-crosslinkable polyolefin article, whereby anextruded, moulded or formed article comprising the materials describedabove is subjected to UV radiation on-line with the extrusion, mouldingor forming operation.

In yet another aspect, the present invention provides a UV-crosslinkablepolymer composition, comprising: (a) a polyolefin selected from one ormore members of the group consisting of polyethylene and polypropylene,and copolymers and terpolymers thereof; (b) cationically polymerizablefunctional groups; and (c) a cationic photoinitiator in an amounteffective to initiate curing of said composition.

In yet another aspect, the present invention provides UV-crosslinkedarticles comprised of a polymer composition, the polymer compositioncomprising: (a) a polyolefin selected from one or more members of thegroup consisting of polyethylene and polypropylene, and copolymers andterpolymers thereof; (b) cationically polymerizable functional groups;and (c) a cationic photoinitiator in an amount effective to initiatecuring of said composition; wherein the article is crosslinked byexposure to UV radiation and possesses a sufficient degree ofcrosslinking such that when the article is heated to a temperature abovethe crystalline melting point of the polyolefin, it is softened but doesnot become liquid.

In yet another aspect, the present invention provides a process forpreparing a UV-crosslinked, thermoset article, comprising: (a) forming ablend comprising: (i) a polyolefin selected from one or more members ofthe group consisting of polyethylene and polypropylene, and copolymersand terpolymers thereof; (ii) cationically polymerizable functionalgroups; and (iii) a cationic photoinitiator in an amount effective toinitiate curing of said composition; (b) melt processing the blend toproduce a melt-processed article having a first set of dimensions; (c)cooling the melt-processed article to a solid state; and (d)crosslinking the melt-processed article by exposure to UV radiation tothereby produce said UV-crosslinked, thermoset article, wherein thecrosslinking imparts thermoset characteristics to the article such that,when the article is heated to a temperature above the crystallinemelting point of the polyolefin, it is softened but does not becomeliquid.

The process may further comprise the steps of:

(e) heating the UV-crosslinked, thermoset article to a first temperatureat which it is softened but not melted; (f) stretching the softenedarticle such that the article is expanded beyond the first set ofdimensions; and (g) cooling the stretched article to a secondtemperature below the temperature at which the article is softened whileholding the article in its stretched form.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Composition

Polyolefin Component:

The polyolefin component is selected from one or more members of thegroup comprising polyethylene and polypropylene, and copolymers andterpolymers thereof.

In one embodiment, the polyolefin component is selected from the groupcomprising polyethylene, copolymers of ethylene and terpolymers ofethylene.

The polyethylene may be selected from the group comprising very lowdensity polyethylene (VLDPE), low density polyethylene (LDPE), linearlow density polyethylene (LLDPE), medium density polyethylene (MDPE),linear medium density polyethylene (LMDPE), high density polyethylene(HDPE) and blends thereof.

The terms HDPE, MDPE and LDPE as used herein are defined in accordancewith the American Society for Testing and Materials (ASTM) D1248standard definitions. LDPE is defined to have a density from 0.910 to0.925 g/cm³, MDPE has a density ranging from 0.926 to 0.940 g/cm³ andHDPE has a density of at least 0.941 g/cm³. The density of VLDPE rangesfrom about 0.880 to 0.910 g/cm³, while the densities of LLDPE and LMDPEgenerally fall within the same ranges as LDPE and MDPE, respectively.The polyethylene includes ethylene homopolymers, as well as copolymersand terpolymers in which ethylene is copolymerized with one or morehigher alpha olefins such as propene, butene, hexene and octene.

The copolymers of ethylene may also be selected from ethylene propylene,ethylene vinyl acetate, ethylene vinyl alcohol, ethylene methylacrylate, ethylene ethyl acrylate, and ethylene butyl acrylate. Theterpolymers of ethylene may also be selected from ethylene methyl, ethylor butyl acrylates with maleic anhydride or glycidyl methacrylate,ethylene propylene diene terpolymers, and ethylene propylene with maleicanhydride or glycidyl methacrylate.

In another embodiment, the polyolefin component is selected from thegroup comprising polypropylene, copolymers of propylene and terpolymersof propylene. The polypropylene may be selected from the groupcomprising predominantly isotactic polypropylene. The polypropyleneincludes propylene homopolymers as well as copolymers and terpolymers ofpropylene with other alpha olefins such as ethylene and butene.

The copolymers and terpolymers of propylene may also be selected frompropylene with maleic anhydride or glycidyl methacrylate, and ethylenepropylene diene terpolymers such as ethylene propylene norbornene.

The polymers comprising the polyolefin component may preferably bemanufactured using metallocene catalysts, also known as single-site,stereo-specific or constrained geometry catalysts, and may also comprisea bimodal molecular weight distribution.

The polyolefin component is added to the composition in an amountranging from 10 to 98 percent by weight, preferably in the range from 50to 95 percent by weight.

Cationically Polymerizable Functional Component:

The component which comprises cationically polymerizable functionalgroups may comprise polymers, such as polyolefins, containingcationically polymerizable functional groups such as glycidylmethacrylate-, epoxy-, oxetane- and vinyl ether-based functionalities.For example, the functional polymers may be selected from polyethyleneor polypropylene homopolymers and copolymers grafted, copolymerized orblended with one or more cationically polymerizable functional groups.Alternatively, the functional component may be one or more additivescomprising functional monomers or oligomers, i.e. monomers or oligomerscontaining cationically polymerizable functional groups. These additivesmay be in the form of solid or liquid additives.

The cationically polymerizable functional groups may be selected fromthe group comprising: glycidyl methacrylates, glycidyl ethers, vinylethers, divinyl ethers, epoxides, diepoxides, oxazolines, oxetanes,epoxy acrylates, epoxy silanes, epoxy siloxanes, and polyols, and blendsthereof.

In one embodiment, the cationically polymerizable functional groups arecovalently bonded to the polyolefin component of the composition,described above. This may typically be accomplished by directcopolymerization of a functional monomer with the olefin monomer ormonomers, or by grafting the functional monomer onto the polyolefinmolecule using a peroxide free-radical initiator such as dicumylperoxide, for example.

In another embodiment, the cationically polymerizable functional groupsare covalently bonded to polymers other than the polyolefin component ofthe composition, wherein the polymers to which the functional groups arebonded are blended with said polyolefin component.

In another embodiment, the cationically polymerizable functional groupsare added as separate functional monomers or oligomers, which may bepreferentially grafted to the polyolefin component prior to, or in-situwith, melt processing of the finished article. A peroxide initiator,such as dicumyl peroxide, may be used to promote the grafting reaction,though grafting may also be initiated as a result of UV irradiation ofthe article. Examples of functional monomers and oligomers includeepoxidized vegetable oils and esters such as epoxidized soybean oil,epoxidized octyl soyate and methyl epoxy lindseedate, epoxidized alphaolefins including those ranging in molecular chain length C₁₀ to C₃₀,epoxidized polybutene, cycloaliphatic epoxides such as3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate andbis-3,4-(epoxycyclohexylmethyl)adipate, epoxy acrylates andmethacrylates such as bisphenol A epoxy diacrylate and aliphatic epoxyacrylates, epoxy silanes, such as γ-glycidoxypropyltrimethoxy silane,oxetanes such as 3-ethyl-3-hydroxymethyl oxetane, and vinyl ethers suchas octadecyl vinyl ether, butanediol divinyl ether, triethyleneglycoldivinyl ether, and vinyl ether terminated esters and urethanes.

The functional component is added in an amount which is sufficient toprovide the composition or a shaped article produced therefrom withthermoset properties, once the composition or article is crosslinked byUV radiation. For example, the cationically polymerizable functionalgroups may be added to the composition in an amount ranging from 0.1 to50 percent by weight, preferably in the range from 1 to 20 percent byweight.

Cationic Photoinitiator:

The cationic photoinitiator may be a radiation-sensitive onium salt, andmay be selected from the group comprising radiation-sensitive diazonium,halonium, iodonium, sulphonium and sulphoxonium salts.

Examples of radiation-sensitive onium salts include aryldiazonium salts,aryliodonium salts, diaryliodonium salts, alkylaryliodonium salts,arylsulphonium salts, triarylsulphonium salts, diarylbromonium salts,triarylselenonium salts, thioxanthonium salts, triarylsulphoxoniumsalts, aryloxysulphoxonium salts, dialkylacylsulphoxonium salts,dialkylphenacylsulphonium salts and dialkyl-4-hydroxyphenylsulphoniumsalts.

In one embodiment, the cationic photoinitiator is selected fromtriarylsulphonium hexafluorophosphate, and diaryliodoniumhexafluoroantimonate.

Alternatively, the cationic photoiniator may be selected from one ormore members of the group comprising iron arene complexes, ferroceniumsalts, thiopyrylium salts, sulphonyloxy ketones, acyl silanes and silylbenzyl ethers.

Further, the cationic photoinitiator may be combined with an organiccarrier solvent such as an alkyl or alkylene carbonate, acetate orpropionate. Examples of these include ethylene carbonate, propylenecarbonate, diethyl carbonate, dimethyl carbonate, ethyl methylcarbonate, butylene carbonate methyl acetate, ethyl acetate, ethylpropionate, and methyl propionate.

The cationic photoinitiator is added in an amount effective to initiateUV-crosslinking of the composition or a shaped article produced from thecomposition. For example, the cationic photoinitiator may be added tothe composition in an amount ranging from 0.1 to 10 percent by weight,preferably in the range, from 0.5 to 5 percent by weight.

Free-radical Polymerizable Functional Component:

The UV-curable composition according to the invention may furthercomprise free-radical polymerizable functional groups such as acrylatesand methacrylates, preferably covalently bonded to the polyolefincomponent of the formulation. Examples include polyolefins modified withacrylates, methacrylates, and glycidyl methacrylates, and polyfunctionalmonomers and oligomers such as acrylates and methacrylates, includingpolyester, polyol, epoxy and polyether acrylates and methacrylates.

The free-radical groups are added in an amount effective to acceleratecuring of the composition or a shaped article produced from thecomposition. For example, the free-radical groups may be added to thecomposition in an amount ranging from 0 to 50 percent by weight,preferably in the range from 1 to 20 percent by weight.

Free-radical Photoinitiator:

The UV-curable polymer composition according to the invention mayfurther comprise a free-radical photoinitiator to increase theinitiation rate of crosslinking and to maximize cure. It will beappreciated that the free-radical photoinitiator may optionally be addedto the composition, whether or not the composition also includes afree-radically polymerizable component.

The free-radical photoinitiator may be selected from one or more membersof the group comprising benzophenones, acetophenones, benzoin ethers,benzils, benzylketals, benzoyl oximes, aminobenzoates, aminoketones,hydroxyketones, ethylamines, ethanolamines, alkylphenones, anthracenes,anthraquinones, anthrachinones, xanthones, thioxanthones, quinones,fluorenones, peroxides, and acylphosphine oxides. Examples offree-radical photoinitiators include benzophenone,2,2-diethoxyacetophenone, 1-hydroxycyclohexylphenyl ketone, benzyldimethylketal, and 2,4,6-trimethylbenzoyldiphenylphosphine oxide.

The free-radical photoinitiator is added in an amount effective toaccelerate curing of the composition or a shaped article produced fromthe composition. For example, the free-radical photoinitiator may beadded to the composition in an amount ranging from 0 to 10 percent byweight, preferably in the range from 0.5 to 5 percent by weight.

Functional Additive:

The UV-curable polymer composition according to the invention mayfurther comprise an effective amount of a functional additive as acrosslinking accelerator, promoter, sensitizer, or chain transfer agent.

The functional additive may be selected from the group comprising monoand polyfunctional acrylates and methacrylates, including polyester,polyol, epoxy and polyether acrylates and methacrylates, allylics,cyanurates, maleimides, thiols, alkoxysilanes, and hydroxyl-containingcompounds such as hydroxyketones, alcohols, diols and polyols. Examplesof specific functional additives include trimethylol propanetriacrylate, trimethylol propane trimethacrylate, tetramethyloltetraacrylate, pentaerythritol triacrylate, ethylene glycoldimethacrylate, triallyl cyanurate, triallyl isocyanurate, vinyltrimethoxysilane, dimercaptodecane, diallyl maleate,N,N-(m-phenylene)-bismaleimide, 1,4-butanediol, ethylene glycol,polypropylene glycol, 1-hydroxy cyclohexyl phenyl ketone, andpolycaprolactone.

The functional additive is added in an amount effective to accelerateand maximize curing of the composition or a shaped article produced fromthe composition. For example, the functional additive may be added tothe composition in an amount ranging from 0.1 to 20 percent by weight,preferably in the range from 0.5 to 5 percent by weight.

Compatibilizers:

The UV-curable polymer composition according to the invention mayfurther comprise an effective amount of a compatibilizer selected fromone or more members of the group comprising: polyethylenes andpolypropylenes; ethylene-propylene copolymers; ethylene-propylene dieneelastomers; crystalline propylene-ethylene elastomers; thermoplasticpolyolefin elastomers; metallocene polyolefins; cyclic olefincopolymers; polyoctenamers; copolymers of ethylene with vinyl acetate,vinyl alcohol, and/or alkyl acrylates; polybutenes; hydrogenated andnon-hydrogenated polybutadienes; butyl rubber; polyolefin ionomers;polyolefin nanocomposites; block copolymers selected from the groupcomprising styrene-butadiene, styrene-butadiene-styrene,styrene-ethylene/propylene and styrene-ethylene/butylene-styrene; andall of the above optionally modified with reactive functional groupsselected from the group consisting of silanes, acrylic acids,methacrylic acids, acrylates, methacrylates, glycidyl methacrylates,epoxies, hydroxyls, and anhydrides.

The compatibilizer is added in an amount effective to enhancemiscibility of the composition components and provide optimum mechanicalproperties of the finished article. For example, the compatibilizer maybe added to the composition in an amount ranging from 1 to 50 percent byweight, preferably in the range from 1 to 20 percent by weight.

Antioxidants and Stabilizers:

The UV-curable polymer composition according to the invention mayfurther comprise one or more antioxidants and heat stabilizers toprevent degradation of the composition during melt processing andsubsequent heat aging of the final product. Examples of suitableantioxidants and heat stabilizers include those classes of chemicalsknown as hindered phenols, hindered amines, phosphites, bisphenols,benzimidazoles, phenylenediamines, and, dihydroquinolines. It shouldalso be noted that these antioxidants and stabilizers, if added inexcessive amounts, may become “radiation scavengers”, acting to limitthe effectiveness of the radiation to induce the desired crosslinkingreaction and the resultant degree of crosslinking obtainable for a givenradiation dosage. Also, the effectiveness of cationic photoinitiatorscan be adversely affected by the presence of basic compounds, such asamines, for example.

The addition of antioxidants and stabilizers is dependent upon therequired degree of thermal stability required in the final article, butthey are typically added in an amount ranging from 0.1 to 5 percent byweight of the total composition.

Foaming Agents:

The UV-curable polymer composition according to the invention mayfurther comprise one or more foaming agents for the preparation offoamed or thermally insulative formulations. Examples of suitablefoaming agents include one or more members of the group comprisingsodium bicarbonate, citric acid, tartaric acid, azodicarbonamide,4,4-oxybis(benzene sulphonyl)hydrazide, 5-phenyl tetrazole,dinitrosopentamethylene tetramine, p-toluene sulphonyl semicarbazide,carbon dioxide, nitrogen, air, helium, argon, aliphatic hydrocarbonssuch as butanes, pentanes, hexanes and heptanes, chlorinatedhydrocarbons such as dichloromethane and trichloroethylene,hydrofluorocarbons such as dichlorotrifluoroethane, and hollowmicrospheres, including glass, polymeric or ceramic microspheres.

The foaming agent is added to the composition in an amount suitable toachieve a desired degree of foaming, which depends somewhat on theintended use of the foamed composition. A typical degree of foaming isin the range from 10 to 50 percent by volume.

Fillers and Flame Retardants:

The UV-curable polymer composition according to the invention mayfurther comprise one or more fillers and/or flame retardants forimproved performance or cost.

Fillers may be selected from one or more members of the group comprisingcalcium carbonate, kaolin, clay, mica, talc, silica, wollastonite,barite, wood fibres, glass fibres, glass, polymer and ceramicmicrospheres, carbon black, nanofillers, and metal oxides such asantimony trioxide, silica and alumina.

Flame-retardants may be selected from one or more members of the groupcomprising halogenated flame-retardants such as aliphatic,cycloaliphatic and aromatic chlorinated and brominated compounds, andhalogen-free flame-retardants such as aluminium trihydrate, organicphosphates, phosphorus-nitrogen compounds, and zinc borate.

The inventors have surprisingly found that antimony trioxide reactssynergistically to give higher levels of crosslinking than if it is notused. The inventors have also found that hindered amines behaveantagonistically and are preferably not added to the composition. Otherfillers and flame retardants have less effect one way or the other oncrosslinking.

The level of filler or flame-retardant added is dependent upon the costand performance requirements of the finished article. In the case ofantimony oxide, preferred levels have been found to be within the range1 to 20 percent by weight.

Process

The composition according to the invention is prepared by first blendingthe aforementioned components. This can be performed either as aseparate step prior to melt processing of the finished article, orsimultaneously with melt processing of the finished article, using amulti-component metering system, for example.

When performed as a separate prior step, the components are preferablymelt blended by a machine specifically designed for that purpose, suchas a continuous single-screw or twin-screw extrusion compounder,kneader, or internal batch mixer.

If it is required to graft the functional component to the polyolefincomponent using a peroxide initiator, for example, this is bestaccomplished as a separate step prior to melt processing and forming ofthe finished article, in an extruder, mixer, or reactor specificallydesigned for the grafting operation. The blended or grafted compositionmay then be pelletized and stored for subsequent melt processing intothe desired finished article.

In the case of extrusion processing, it is preferable that thecomponents are added as pelleted solids. This is typically the suppliedform of the polyolefin components or polymeric compatibilisers describedabove. However, since many of the additives mentioned above, andparticularly the antioxidants, stabilizers, fillers andflame-retardants, are naturally occurring powders, it is preferable thata pelleted masterbatch be prepared beforehand using a compatible polymeras the carrier or binder for the additives. Alternatively, it may bepossible to combine the compounding and extrusion processing operationinto a single step if the extruder used is a so-called compoundingextruder, such as a twin-screw extruder, or kneader. Care is requiredhere to ensure that full dispersion of the additives has occurred beforethe material reaches the extrusion forming die, and that any melt flowfluctuations have been eliminated. A gear pump or static mixing deviceinstalled between the end of the extruder screw and the entrance to theextruder die may also be required.

In cases where the functional monomers or oligomers, photoinitiators andcrosslinking accelerators are liquids, it is preferable to mix thesedirectly with the molten polymer composition. For example, in asingle-screw extrusion operation this would be accomplished using ascrew design having a decompression zone approximately midway along itslength, at which point the liquid additives are injected into thepolymer melt stream. Alternatively, the liquid additives may be coatedonto the polymer pellets in a multi-component blender installed abovethe main feed port of the extruder. Another method of incorporatingliquid additives would be to first imbibe them into a porous polymericcarrier, in which case they can then be effectively handled in the samemanner as a solid, pelleted polymer.

In all cases it is important to homogeneously distribute thephotoinitiators and accelerators within the polymer melt and to minimizeloss of these additives through volatilization. The design of theextruder screw is important to achieve proper mixing and conveying ofthe components, and it may be necessary to incorporate barrier flightsand mixing elements. Additionally, a static mixing attachment may beinserted between the end of the screw and the die. Alternatively, atwin-screw extruder having separate and interchangeable screw elementsmay be used.

Melt processing and forming of the composition is performed by extrusionand moulding techniques commonly used in the industry. Examples ofextruded articles include pipes, pipe coatings, sheet, tubing, foams,and electrical insulation. In some preferred embodiments, thecomposition may be co-extruded or laminated with other materials ofsimilar or dissimilar compositions to form laminate structures havingdiscrete but intimately bonded layers, with each layer having differentfunctional properties. For example, an adhesive-coated polymer sheet canbe produced by co-extruding or laminating the composition with anadhesive. In other examples, the composition may be laminated with lessexpensive or non-crosslinkable layers, or it may be extruded atop acorrosion-protection layer, or layers, of a steel pipe thereby providinga multilayer pipe coating with a UV crosslinkable top layer. Moldedarticles can be produced by injection, compression or blow molding, andexamples include electrical insulating articles such as end-caps, spliceconnectors, and break-out boots.

Once formed, the article is crosslinked by UV radiation. The inventionallows that this step be accomplished at the same time as, and on linewith, the processing and forming step after the material has solidifiedor crystallized. For example, it is possible to install the UV radiationsource immediately after the sizing and cooling operation on anextrusion line, but before the final product wind-up station. Theproduct does not therefore require a separate, off-line crosslinkingstep subsequent to the processing or forming operation, therebysignificantly reducing processing costs and improving product throughputand manufacturing plant capacity.

Crosslinking is the formation of permanent covalent bonds betweenindividual polymer chains which act to bind the polymer chains togetherand prevent them from irreversibly separating during subsequent heating.It is this crosslinked structure which, while retaining the elastomericnature of the material, renders the material thermoset and resistant tomelting which, in turn, is a desirable property for producingheat-shrinkable articles, as discussed below. Crosslinking also providesthe article with excellent thermal and hot deformation resistance,allowing it to maintain mechanical toughness and integrity at highservice temperatures.

The UV radiation source, or sources, comprises a lamp, or a series oflamps, and reflectors positioned along the length above and/or below, orcircumferentially around, the formed article. The lamps should emitradiation in the wavelength range 100 to 500 nanometers and moreparticularly in the range 200 to 400 nanometers. The emission spectrumof the UV source should match the absorption spectrum of the UVphotoinitiator as closely as possible to maximize the generation ofphotoinitiating species. Medium to high pressure mercury vapour lampsare most commonly used, typically either electric arc or microwavedischarged. Rare gas, such as xenon, lamps can also be used. In the caseof mercury lamps, the addition of metal halides can intensify the outputof certain specific wavelengths. In addition to the wavelength, otherfactors to consider for optimum irradiation are the intensity of the UVradiation, dictated by the energy output of the lamp (typically 30 to200 W/cm), the geometry of the lamp reflectors (typically elliptical orparabolic), the distance of the article from the UV source, and thedosage, which is also related to the rate of conveyance of the articlethrough the UV radiation.

As mentioned above, crosslinked articles produced according to theinvention, such as sheet, tubing and moulded shapes, can be renderedheat-shrinkable since they exhibit the thermoset property of not meltingwhen heated to a temperature close to or above the crystalline meltingpoint of the highest melting point component. This is important becausethe crosslinked structure allows the article to be stretched withminimal force and without melting, and to retain its mechanicalintegrity, when heated to this temperature. The hot article is fixed inthis stretched state by rapidly cooling it to below the crystallinemelting point while holding the article in its stretched position, there-formed rigid crystalline regions of the polymeric components of thematerial preventing the article from spontaneously recovering to itsoriginal dimensions. Stretching of the article can be accomplished bymechanical, pneumatic or hydraulic means. Cooling the article in itsstretched state may be accomplished by a cooling medium such as air,water or other heat-transfer medium.

Subsequent re-heating of the stretched article above the melting pointwill cause the crystalline regions to re-melt and the structure toelastomerically recover to its original unstretched dimensions. Thecrosslinked structure provides the initial recovery force and againensures that the article does not melt and that it maintains itsmechanical integrity.

The heating, stretching and cooling steps thus described for theproduction of heat-shrinkable articles may be accomplished either as asubsequent separate operation, or on-line with the processing, formingand UV crosslinking operation described earlier.

The degree of crosslinking is quantified through gel fraction and hottensile strength measurements. The gel fraction is the quantity ofcrosslinked polymer remaining after any uncrosslinked fraction has beenremoved by refluxing in hot solvent, such as decahydronaphthalene orxylene. This gives information on the extent or amount of thecrosslinked network but not the density or strength of the network. Ahigh gel fraction does not necessarily indicate robust performance ofthe crosslinked material above the melting point. For this, ameasurement of the tensile strength above the melting point of thepolymer is necessary, since crosslinking is primarily restricted to theamorphous regions of the polymer. The hot tensile strength, therefore,provides information on the mechanical behaviour of the material abovethe melting point and provides insight into properties such as theheat-recovery characteristics and hot deformation resistance of thecrosslinked product.

The invention is further illustrated by way of the following examples:

EXAMPLE 1a

A functional ethylene terpolymer (E-MA-GMA) containing 24% by weightmethyl acrylate and 8% by weight glycidyl methacrylate and of density0.94 g/cm³ and melt flow index 6 dg/min. was blended with an ethylenepropylene diene terpolymer (EPDM) of density 0.908 g/cm³ and melt flowindex 1.0 dg/min., a cationic photoinitiator comprisingtriarylsulphonium hexafluorophosphate in propylene carbonate, afree-radical photoinitiator comprising 1-hydroxy-cyclohexylphenylketoneand benzophenone, and a trimethylolpropane triacrylate crosslinkingpromoter, in the amounts shown in Table 1. The liquid cationicphotoinitiator, free-radical photoinitiator and crosslinking promoterwere imbibed into a porous HDPE carrier at a ratio of approximately 2:1to aid blending with the polymeric components. Blending was accomplishedwith a tumble blender, ribbon blender, high-speed blender, ormulti-component feeding system.

The blended components were fed through a 24:1 L/D single-screwextruder, equipped with a polyethylene mixing screw and single-layersheet die, and extruded at a melt temperature of approximately 140° C.into sheet of thickness approximately 1.2 mm. The extruded sheet wasfixed to the required dimensions of width, thickness and orientation bypassing it through a chilled, 3-roll calendering stack. The cooled,solidified sheet was then conveyed at a distance of 5 cm. beneath, andat a speed of 200 cm/min. through, a UV radiation source comprising aType D medium pressure mercury lamp operating at a wavelength of 250 to400 nm. and about 80 W/cm intensity.

The UV crosslinked sheet was then tested after 24 hours to determine thedegree and density of crosslinking achieved, and for the mechanicalproperties indicated in Table 2.

The UV crosslinked sheet was further re-heated to a temperature ofapproximately 150° C. and then stretched by approximately 30% in lengthusing a mechanical stretcher. Whilst in this stretched state, the sheetwas rapidly cooled to below the crystalline melting points of thepolymers comprising the composition in order to fix the sheet at thestretched dimensions. The heat-shrinkable sheet thus prepared wassubsequently laminated with a layer of hot-melt adhesive andheat-recovered over a welded steel pipe joint.

EXAMPLE 1b

The molten extruded sheet of Example 1a was wrapped circumferentiallyaround the surface of a rotating steel pipe previously coated with anepoxy-based corrosion protection layer, and UV crosslinked using aseries of UV lamps positioned circumferentially around the pipe.

EXAMPLE 1c

The composition of Example 1a was extruded through an annular die, thetube or pipe thus formed being cooled and UV crosslinked as describedabove. The crosslinked tube or pipe may subsequently be renderedheat-shrinkable by re-heating, stretching, and cooling as describedabove.

EXAMPLE 1d

The composition of Example 1a was compression moulded into an electricalcable end-cap, cooled and then UV crosslinked. The crosslinked end-capwas subsequently re-heated, stretched and cooled to render the end-capheat-shrinkable.

EXAMPLE 2

This example follows Example 1 except that the crosslinking promoter iseliminated from the composition.

EXAMPLE 3

This example follows Example 1 except that the EPDM component isreplaced by a HDPE of density 0.947 g/cm³ and melt flow index 0.28dg/min.

EXAMPLE 4

This example follows Example 1 except that the free-radicalphotoinitiator and crosslinking promoter are eliminated from thecomposition.

EXAMPLE 5

This example follows Example 1 except that the amount of E-MA-GMA isreduced and the EPDM component is replaced by a HDPE of density 0.947g/cm³ and melt flow index 0.28 dg/min.

EXAMPLES 6, 7 and 8

These examples examine the effect of incorporating antimony trioxideinto the composition. With the exception of the antimony trioxideaddition, Examples 6 and 7 follow Examples 1 and 4, respectively,whereas Example 8 follows Example 2, but also eliminates the cationicinitiator.

EXAMPLES 9 and 10

Examples 9 and 10 follow Example 1 except that the EPDM component isreplaced by a HDPE of density 0.947 g/cm³ and melt flow index 0.28dg/min, and the E-MA-GMA is replaced by an epoxy-acrylate oligomer inorder to examine the effect of incorporating the cationicallypolymerizable functional component as a separate oligomer. Example 10differs from Example 9 in that it also includes dicumyl peroxide as agrafting initiator for said oligomer.

Examples 2-10 are included for comparative purposes. The compositionswere prepared by mixing the components indicated in Table 1 using aBrabender laboratory internal mixer set at a temperature ofapproximately 200° C. The mixed compositions were then pressed intoplaques of approximate thickness 1.0 mm., and UV crosslinked asdescribed in Example 1. All amounts shown in Table 1 are in parts byweight of the respective compositions.

TABLE 1 Compositions Ingredient Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex.7 Ex. 8 Ex. 9 Ex. 10 E-MA-GMA 50 50 50 50 25 45 45 45 EPDM 50 50 50 4545 45 HDPE 50 75 100 100 Cationic 1 1 1 1 1 1 1 1 1 InitiatorFree-Radical 1.5 1.5 1.5 1.5 1.5 Initiatior Crosslinking 1 1 1 1Promoter Antimony 10 10 10 Trioxide Epoxy- 2 2 Acrylate Dicumyl 0.08Peroxide

TABLE 2 Properties Ex. Property Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex.7 Ex. 8 Ex. 9 10 Gel Fraction (%) 73 74 47 69 30 71 55 49 4 10 HotTensile Strength 68 58 40 97 28 100 110 6 1 9 @ 200° C. and 100%Elongation (psi) Ultimate Tensile 14 17 15 15 23 11 15 6 33 29 Strength@ 23° C. (psi) Ultimate Elongation 300 300 310 300 400 320 310 570 840280 @ 23° C. (%)

1. An ultraviolet-crosslinked article comprised of a polymercomposition, the polymer composition comprising: (a) a polyolefinselected from one or more members of the group consisting ofpolyethylene, polypropylene, copolymers of polyethylene, terpolymers ofpolyethylene and copolymers of polypropylene, wherein the polyolefinincludes cationically polymerizable functional groups covalently bondedto the polyolefin; (b) a cationic photoinitiator in an amount effectiveto initiate curing of said composition; and (c) free-radicalpolymerizable functional groups in an effective amount to acceleratecuring of said composition, wherein the free-radical polymerizablefunctional groups are covalently bonded to the polyolefin; wherein thepolymer composition is crosslinked by exposure to ultraviolet radiationand possesses a sufficient degree of crosslinking such that when thearticle is heated to a temperature above the crystalline melting pointof the polyolefin, it is softened but does not become liquid.
 2. Theultraviolet-crosslinked article of claim 1, wherein the polyolefin isselected from the group consisting of polyethylene, copolymers ofpolyethylene and terpolymers of polyethylene.
 3. Theultraviolet-crosslinked article of claim 2, wherein the polyethylene isselected from one or more members of the group consisting of very lowdensity polyethylene (VLDPE), low density polyethylene (LDPE), linearlow density polyethylene (LLDPE), medium density polyethylene (MDPE),linear medium density polyethylene (LMDPE) and high density polyethylene(HDPE).
 4. The ultraviolet-crosslinked article of claim 2, wherein thecopolymers of polyethylene are selected from one or more members of thegroup consisting of ethylene-propylene, ethylene-butene,ethylene-hexene, ethylene-octene, ethylene vinyl acetate, ethylene vinylalcohol, ethylene methyl acrylate, ethylene ethyl acrylate, and ethylenebutyl acrylate.
 5. The ultraviolet-crosslinked article of claim 2,wherein the terpolymers of polyethylene are selected from one or moremembers of the group consisting of ethylene-methyl acrylate-maleicanhydride terpolymer; ethylene-methyl acrylate-glycidyl methacrylateterpolymer; ethylene-ethyl acrylate-maleic anhydride terpolymer;ethylene-ethyl acrylate-glycidyl methacrylate terpolymer; ethylene-butylacrylate-maleic anhydride terpolymer; ethylene-butyl acrylate-glycidylmethacrylate terpolymer; ethylene-propylene-diene terpolymers;ethylene-propylene-maleic anhydride terpolymers; andethylene-propylene-glycidyl methacrylate terpolymers.
 6. Theultraviolet-crosslinked article of claim 1, wherein the cationicallypolymerizable functional groups originate from a reactive compoundselected from the group consisting of glycidyl methacrylates, glycidylethers, vinyl ethers, divinyl ethers, epoxides, diepoxides, oxazolines,oxetanes, epoxy acrylates, epoxy silanes, epoxy siloxanes, polyols andcombinations thereof.
 7. The ultraviolet-crosslinked article of claim 1,wherein the polyolefin comprises an ethylene-methyl acrylate-glycidylmethacrylate terpolymer.
 8. The ultraviolet-crosslinked article of claim1, wherein the polymer composition further comprises a compatibilizerselected from the group consisting of: polyethylenes; polypropylenes;ethylene-propylene copolymers; ethylene-propylene-diene elastomers;crystalline propylene-ethylene elastomers; thermoplastic polyolefinelastomers; metallocene polyolefins; cyclic olefin copolymers;polyoctenamers; ethylene-vinyl acetate copolymers; ethylene-vinylalcohol copolymers; ethylene-alkyl acrylate copolymers; polybutenes;hydrogenated polybutadienes; non-hydrogenated polybutadienes; butylrubber; polyolefin ionomers; polyolefin nanocomposites;styrene-butadiene block copolymers; styrene-butadiene-styrene blockcopolymers; styrene-ethylene/propylene-styrene block copolymers;styrene-ethylene/butylene-styrene block copolymers; and combinationsthereof.
 9. The ultraviolet-crosslinked article of claim 8, wherein thecompatibilizer is functionalized by reaction with a compound selectedfrom the group consisting of a silane, acrylic acid, methacrylic acid,acrylate, methacrylate, anhydride and combinations thereof.
 10. Theultraviolet-crosslinked article of claim 8, wherein the compatibilizeris selected from the group consisting of: polyethylenes,ethylene-propylene copolymers and ethylene-propylene-diene elastomers.11. The ultraviolet-crosslinked article of claim 10, wherein thecompatibilizer is an ethylene-propylene-diene elastomer.
 12. Theultraviolet-crosslinked article of claim 1, wherein the polyolefincomprises from 10-98 percent by weight of the composition.
 13. Theultraviolet-crosslinked article of claim 12, wherein the polyolefincomprises from 50-95 percent by weight of the composition.
 14. Theultraviolet-crosslinked article of claim 8, wherein the compatibilizercomprises from 1-50 percent by weight of the composition.
 15. Theultraviolet-crosslinked article of claim 1, wherein the free-radicalpolymerizable functional groups are selected from one or more members ofthe group consisting of acrylates and methacrylates.
 16. Theultraviolet-crosslinked article of claim 1, wherein the polymercomposition further comprises a free-radical photoinitiator in aneffective amount to accelerate curing of said composition.
 17. Theultraviolet-crosslinked article of claim 16, wherein the free-radicalphotoinitiator is selected from one or more members of the groupconsisting of benzophenones, acetophenones, benzoin ethers, benzils,benzylketals, benzoyl oximes, aminobenzoates, aminoketones,hydroxyketones, ethylamines, ethanolamines, alkylphenones, anthracenes,anthraquinones, anthrachinones, xanthones, thioxanthones, quinones,fluorenones, peroxides, and acylphosphine oxides.
 18. Theultraviolet-crosslinked article of claim 1, wherein the polymercomposition further comprises a filler, and wherein the filler comprisesa metal oxide.
 19. The ultraviolet-crosslinked article of claim 18,wherein the metal oxide comprises antimony trioxide.
 20. Theultraviolet-crosslinked article of claim 1, wherein the article isheat-shrinkable.
 21. The ultraviolet-crosslinked article of claim 1,wherein the article is a shaped article.
 22. The ultraviolet-crosslinkedarticle of claim 1, wherein the article is a coating or insulatingmaterial.
 23. The ultraviolet-crosslinked article of claim 1, whereinthe article is selected from the group consisting of heat-shrinkablepipe joint protection sleeves, wire and cable insulation or jacketing,pipe coatings, sheet material, tubing, end caps, break-out boots, andmulti-laminate sheet, tubing or pipe coating.